headerbar: match pixel height for image buttons

- chasing the single pixel difference. of couse solving
  this with padding is a little futile in that the font
  size will break it.

  https://bugzilla.gnome.org/show_bug.cgi?id=721622
osd toolbar: flat and subtle backdrop

https://bugzilla.gnome.org/show_bug.cgi?id=725748
Fix dark pixels in outer top corners of leftmost and rightmost linked buttons

https://bugzilla.gnome.org/show_bug.cgi?id=704735